An ultra high purity supply of inert gas, particularly argon, has become essential in the manufacture of large scale integrated circuits. Semiconductor manufacturers utilize commercial purifiers to remove impurities in the argon stream to less than 10 parts per billion (ppb). Some of the more important impurities removed by these purifiers include O.sub.2, H.sub.2 O, CO, H.sub.2, CO.sub.2, CH.sub.4, and N.sub.2. Continuous monitoring of the inert gas stream under continuous flow conditions to assure that the gas stream purity continues to meet its stringent specifications is mandatory. With the exception of nitrogen, analytical instruments presently exist which are able to satisfy the required &lt;10 ppb mimimum detection limit (MDL). Nitrogen is also the most likely impurity to "break through" the commercial purifier system and contaminate the high purity gas stream.
Currently the only method for continuous monitoring of nitrogen at low concentration levels in a high purity argon gas stream is emission spectroscopy. In emission spectroscopy the gases are excited in a gaseous discharge to produce optical emission lines characteristic of each gas in the gas stream. The emission line for nitrogen is then isolated and analyzed to measure its intensity in order to quantify its concentration. To date, this method is however limited to a minimum detection limit (MDL) for nitrogen in the range of 0.1-0.5 ppm(100-500 ppb).
The conventional emission spectrometer employs a dielectric quartz tube having two electrodes extending therefrom to which an alternating electric field is applied at a high potential sufficient to cause an electric discharge. The gas sample is fed into the tube under continuous flow conditions and is excited by absorption of energy during the electric discharge. This results in the emission of radiant energy as the gas molecules drop from an elevated energy level to lower energy levels. The wavelengths of this emission are characteristic of the gas components excited by the absorption and release of energy. By filtering out unwanted wavelengths the intensity of the emission of any gas in the gas stream can be measured. In an argon gas stream, the concentration level of an impurity gas such as nitrogen can be measured by optically isolating light at the strongest characteristic wavelength for nitrogen i.e. at 337.1 nm and converting the separated optical signal to a corresponding electrical signal.
In conventional emission spectroscopy, the radiated output signal from the electric discharge source is modulated to produce an alternating signal using a mechanically rotating wheel sometimes colloquially referred to as a "chopper". The chopper, thus used to modulate the optical signal output from the electric discharge tube, produces a desired modulation frequency of e.g. 510 Hz. The modulated signal is then filtered to isolate the emission line 337.1 nm which is detected at the modulated frequency using signal electronics which includes a tuned amplifier to selectively amplify the 510 Hz modulated frequency signal and to reject other frequencies. A chopper has been used in emission spectroscopy for modulating the optical signal output of the silent electric discharge tube from its early inception. The function and need for a chopper in emission spectroscopy is described in detail in U.S. Pat. No. 3,032,654 issued May 1, 1962.